1. Field of the Invention
The present invention relates to an oxygen scavenger for use in containers such as bottles and cans or closures thereof, and more particularly relates to an oxygen scavenger which is to be attached to containers or closures and which can increase the preservation stability of substances stored in containers by absorbing oxygen present in the containers, for example, for storage of beverages, liquids, seasonings, foods, chemicals and so forth (hereinafter, referred to as "beverages and so forth") such as a beer, sake, shochu (Japanese vodka), wine, vodka, juice, soy sauce, mirin (Japanese seasoning), miso, various sea foods and various chemicals which are deteriorated by oxygen and thereby changing their tastes or flavors or reducing their qualities.
2. Description of the Prior Art
In preservation of the beverages and so forth which are liable to be affected by oxidation, there is a problem that the beverages and so forth are oxidized by a small amount of oxygen present in a container used for storage and/or the beverages and so forth themselves and their tastes or flavors change during preservation in the container. To solve this problem, for example, vacuum packing, nitrogen replacement and use of oxygen absorbents are known as methods for removing oxygen present in containers. In particular, the following methods (A) and (B) are well known as methods for reducing the amount of oxygen present in a container used for charging beverages or liquids therein.
(A) gas replacement method (sealing method and non-sealing method) and preevacuation (single system and double system) for reducing the amount of oxygen present in a container before charging operation.
(B) foaming method (jet foamer method and supersonic wave method) and gas replacement method (capper method, seamer method and precapping method) for reducing the amount of oxygen present in the headspace of a container after the charging operation.
For example, in the beer industry, since oxygen oxidizes the components dissolved in beer and thereby reduces the flavor of the beer, it is said that the shorter is the period from the time of production to the time of drinking, the better is the taste of the beer. Therefore, proposed are the jet foamer method for injecting a small amount of pressurized water to foam the beer after charging the beer as a mechanical deoxygenation method and the oxygen absorption method due to ascorbic acid, glucose oxidase or yeast as a chemical deoxygenation method.
Alternatively, methods for attaching an oxygen absorbent to a cap of a beer container are also proposed. These methods can be classified into the following three groups in accordance with whether the oxygen absorbent is contained in a resin and/or whether the oxygen absorbent is fixed to a crown of the beer container via a barrier member to avoid the direct contact of the oxygen absorbent with the beer.
(1) The methods for containing an oxygen absorbent in a resin are disclosed in, for example, Japanese Patent Laid-Opens HEI-1-308781 and HEI-1-315438. The former publication discloses a method for fixing the mixture of a low density polyethylene, ascorbic acid and/or sodium sulfite, a lubricant (sodium dodecyl sulfate) and an antioxidant onto the back surface of a crown, thereby increasing the life of a beer. The latter publication discloses a method for attaching a polypropylene containing ascorbic acid to a crown, thereby increasing the preservation stability of a beer. Further, European Patent No. 305005 discloses that the sheet obtained by fixing a dried yeast in a slurry of molten paraffin, applying this slurry at a constant thickness and heat treating the applied slurry is effective to increase the preservation stability of a beer.
(2) The methods for fixing an oxygen absorbent are disclosed in, for example, Japanese Utility Model Laid-Opens SHO 55-161858 and SHO 56-38056 and U.S. Pat. Nos. 4,287,995, 4,421,235 and 4,756,436. These methods aim to prevent the contact of the stored substance with the oxygen absorbent by covering the oxygen absorbent with a sheet permeable to oxygen but impermeable to water and to perform an efficient oxygen absorption. For this sheet, a polyethylene, polypropylene or polyphloroethylene sheet having a pore size of 0.01 to 0.45 .mu.m can be used. This sheet may be coated with a silicone resin or fluorocarbon in order to increase its water repellent property. The above publications propose iron, iron sulfate, iron chloride, dithionate, dithionite, ascorbic acid and its salt, catechol, hydroquinone, pyrogallol, Rongalit, copper-amine complex and so forth as oxygen absorbents. PCT W089/12119 discloses a method wherein the oxygen absorbent insoluble to water made by applying metallic complex of polyalkylamine onto silica gel is fixed to the inside of a crown of a beer bottle or the inside of a beer can together with a gas permeable barrier member.
(3) Japanese Patent Laid-Open SHO 57-194959 discloses a method for attaching an oxygen absorbent to a crown, wherein the oxygen absorbent is contained in a resin and further a barrier member is provided. In this method, the oxygen absorbent is dispersed in an elastomer, and the mixture is fixed to the inside of the crown. A covering membrane composed of a polymer permeable to oxygen and water vapor but impermeable to water is provided on the oxygen absorber.
Other conventional methods or oxygen scavengers other than those described in the above (1), (2) and (3) are also known. Japanese Patent Laid-Open SHO 56-2164 discloses an oxygen scavenger wherein a composite wrapping material comprising two layers including a layer of a non-woven fabric and a layer having a permeability to gaseous oxygen but impermeable to water is used to the wrapping member for an oxygen absorbent. Japanese Patent Laid-Open HEI 1-167079 discloses an oxygen absorbable wrapping material holding an oxygen absorbent between an outer layer of poly-4-methylpentene and an inner layer of a resin capable of being easily heat sealed and having oxygen permeability. Japanese Patent Laid-Open SHO 62-122569 discloses a method wherein oxygen is concentrated into a limited space defined by a selective oxygen permeable membrane, and the concentrated oxygen is efficiently absorbed by an oxygen absorbent provided in another space.
In the above-described conventional technologies, however, there are the following problems.
A first problem is that it is difficult to screen an oxygen absorbent from the substance stored in a container while maintaining a sufficient oxygen permeability to the oxygen absorbent. The method for merely containing an oxygen absorbent in a resin cannot prevent the contact of the oxygen absorbent with the stored substance. To solve this problem, a method for fixing the oxygen absorbent via a barrier member is proposed. With this barrier member, a sheet (non-porous membrane), a porous membrane and a porous membrane applied with water repellent finishing are used. In the case of the sheet of the non-porous membrane, although a material having a high oxygen permeability is selected as the material for the sheet, the sheet requires a thickness greater than that of .mu.m order to ensure its mechanical properties. However, if the non-porous membrane having such a thickness is employed, its oxygen permeability is greatly reduced. On the other hand, in the case of the porous membrane, although the membrane has a high oxygen permeability because the membrane has pores with a pore size of generally greater than .mu.m order, it is difficult that the membrane completely screens the oxygen absorbent from the stored substance. Particularly in the system where the inside of a container is in a pressurized condition during preservation such as preservation of a beer or in the system where the substance is a liquid with low surface tension such as alcohlic beverages, this problem appears significantly. In order to solve this problem, a method for coating a water-repellent polymer on the porous membrane is proposed. In this coating method, however, arises another new problem that the coating polymer is impregnated into the pores of the porous membrane, thereby reducing its oxygen permeability. Namely, since it is difficult to make the non-porous barrier sheet thinner or control the pore size of the porous membrane to a fine size in these methods, an effective screening while maintaining a sufficient oxygen permeability cannot be achieved.
A second problem is that it is difficult to conserve an oxygen scavenger containing an oxygen absorbent for a long period of time. In many cases, oxygen scavengers are placed in the atmosphere from their production to their attachment. Therefore, the oxygen scavengers absorb the oxygen in the atmosphere during conservation. If an oxygen scavenger has already consumed most of its oxygen absorption ability at the time of attachment, the oxygen scavenger cannot be employed in practical use. In order to solve this problem, a method for giving a function to an oxygen absorbent to stop the oxygen absorption during conservation and to start the oxygen absorption after attachment by giving a certain stimulus to the oxygen absorbent is proposed. For example, PCT W089/12119 discloses a method wherein polyalkylamine fixed to silica gel, which is an oxygen absorbent precursor, is mixed with a metallic salt, and as the result that the metallic salt is dissolved by the water component contained in the employed system and the dissolved metallic salt is taken into the polyalkylamine, the oxygen absorbent can exhibit its oxygen permeability. In this method, however, preparation of polyalkylamine fixed to silica gel is troublesome and the oxygen absorbent cannot exhibit a sufficient oxygen permeability under a low moisture condition.
As the methods other than those for attaching an oxygen scavenger or oxygen absorbent to a crown, Japanese Patent Laid-Opens SHO 56-2164 and HEI 1-167079 disclose a method for providing a resin layer impermeable to water directly on a non-woven fabric or a resin material having a good heat sealability and an oxygen permeability. In this method, however, it is difficult to form a homogeneous resin layer with no defect. The resin layer is required to be fairly thick in order to form a homogeneous resin layer with no defect, and therefore, it is impossible to obtain an oxygen scavenger having a high oxygen absorption rate. Japanese Patent Laid-Open SHO 62-122569 discloses a method for attaching a closure having an oxygen permeability to a sealable container in order to increase the preservation stability of a sauce for noodles. In this method, however, a space for storing the concentrated oxygen permeated through a selective oxygen permeable membrane and a space partitioned by a gas permeable supporting member for holding an oxygen absorbent therein are both necessary. Therefore, in this method, a high level production technology is required for the oxygen scavenger, and it is difficult to make the oxygen scavenger small and to obtain a sufficient oxygen permeability thereof. Moreover, although this method uses a polysiloxane-based selective oxygen permeable membrane produced by Monsanto Corporation (U.S. company) which is formed as a hollow fiber like oxygen permeable membrane wherein a non-porous membrane is formed on a porous layer (J. M. S. Henis, M. K. Toripodi, Journal of Membrane Science, 8, 233-246 (1981)), its oxygen permeability is poor and a sufficiently high oxygen absorption ability of the oxygen scavenger using such a membrane cannot be obtained.